Except for the Camlinternal* modules and the new Stdlib module, all
modules in the stdlib now compile to Stdlib__<module>.
Pervasives is renamed to Stdlib and now contains a list of aliases
from the long names to the short ones, so that from inside and outside
the stdlib we can refer to the standard modules as just List or
Stdlib.List rather than Stdlib__list.
In order to avoid printing the long names in error messages and in the
toplevel, the following heuristic is added to Printtyp: given a path
Foo__bar, if Foo.Bar exists and is a direct or indirect alias to
Foo__bar, then prefer Foo.Bar.
A bootstrap step was required to replace Pervasives by Stdlib as the
module opened by default.